The remote gas process is the process to chemically convert natural gas at locations remote from world markets to ultrahigh quality refinery feed stocks or finished products. These products can be shipped, stored and consumed in conventional facilities. Liquefied natural gas has been the method of choice for many years for recovering and shipping remote natural gas. Other alternatives are conversion to methanol, gasoline or ammonia. The high value products, compared to the alternatives, make the remote gas process a very viable process (even though the thermal efficiency is lower) and the subject of many recent studies. The first commercial plant is the Shell Middle Distillate Synthesis plant started in May 1993 in Bintulu, Sarawak, in eastern Malaysia. In this case, natural gas is converted to synthesis gas using non-catalytic partial oxidation and the synthesis gas is converted via Fischer-Tropsch to premium middle distillate products including wax and specialty chemicals. This is a 500,000 metric ton per year facility. Another remote gas process is Exxon's Advanced Gas Conversion process where syngas is produced from combined catalytic partial oxidation/reforming as feed stock to their heavy paraffin synthesis Fischer-Tropsch step followed by hydroisomerization to produce white oil.
The remote gas process normally requires a syngas step in which oxygen from an air separation unit is used to partially oxidize the natural gas, sometimes with steam methane reforming. The air separation unit is typically a conventional low pressure cycle with essentially no air separation unit nitrogen utilization. The Bintulu remote gas process unit has such an air separation unit.
It is the object of this invention to lower the capital investment of the air separation unit compared to common practice or to lower the capital investment of the total remote gas process facility through integration schemes with the air separation unit. It is asserted that an elevated pressure air separation unit may be defined as having feed air compressed between 8 and 30 Bar(a) [800 and 3,000 kPa(absolute)] and preferably between 12 and 18 Bar(a) [1,200 and 1,800 kPa(absolute)]. There is a prevailing industry-wide belief that elevated pressure air separation unit cycles only make sense in cases where the available high pressure nitrogen is utilized at high pressures, such as gas turbine integrations for power generation. Industry has not yet come to realize that, since power efficiency is not of utmost importance for remote gas process, elevated pressure air separation unit cycles are the low cost option, even if high pressure nitrogen is not utilized at high pressure.
A second objective is to more effectively utilize the excess energy (usually in the form of steam) available in the remote gas process.
U.S. Pat. No. 4,888,131 (Goetsch, et al.) and U.S. Pat. No. 5,160,456 (Lahn, et al.) both describe the remote gas process syngas generation process, but make no mention of the air separation unit or air separation unit integration. Further information on the Exxon remote gas process is also taught in a paper by Ansell, L. L., Eisenberg, B. and Bauman, R. F., entitled "Liquid Fuels from Natural Gas--An Update of the Exxon Process", presented at the Council On Alternate Fuels, Apr. 26-29, 1994.
A paper by Tijm, P. J. A., Marriott, J. M., Senden, M. M. G., van Wechem, H. M. H., entitled "Shell Middle Distillate Synthesis The Process, The Products, The Plant", presented at the Council On Alternate Fuels, Apr. 26-29, 1994, gives a summary of the Shell remote gas process, but does not mention any details on the air separation unit.
U.S. Pat. No. 5,251,451 (Agrawal, et al.) describes a typical elevated pressure air separation unit and the integration with gas turbines, but does mention remote gas process.
U.S. Pat. No. 5,081,845 (Allam, et al.) describes an elevated pressure air separation unit with high pressure nitrogen expanded for shaft power and/or refrigeration. Further, the patent teaches to heat high pressure nitrogen with available heat energy and to expand this stream for additional shaft power. It also teaches integration with integrated gasification combined cycle. Power efficiency may be improved by introducing the high pressure nitrogen to the gas turbine combustor. No mention is made of the remote gas process.
U.S. Pat. No. 5,388,395 (Scharpf, et al.) teaches the use of an elevated pressure air separation unit with high pressure nitrogen expanded for shaft power and refrigeration. It also teaches blending cool expanded nitrogen with the gas turbine inlet air to improve shaft power, and also to first saturate the high pressure nitrogen with water before expanding. It mentions integration with an integrated gasifier combined cycle by having the expanded nitrogen provide refrigeration for inlet gas turbine air, but there is no mention of remote gas process.
Waste nitrogen chilling towers have been commonly used in air separation units for years to provide chilled water for feed air cooling, but have not been used as proposed.
In summary, examples have been provided describing the remote gas process and examples have been provided describing the elevated pressure air separation unit and methods of utilizing air separation unit high pressure nitrogen for improved power efficiency, but nowhere is described the use of an elevated pressure air separation unit in conjunction with remote gas process. Conventional thinking for those skilled in the remote gas process art and the air separation unit art is that elevated pressure air separation unit cycles are only economical for gas turbine power generation cycles where high pressure nitrogen is introduced to the gas turbine combustor and that the air separation unit of choice is the low pressure air separation unit as demonstrated in the Bintulu project. The point being overlooked is that for a remote gas process, power is valued comparatively low and low capital expenditures are very important. Also overlooked is the fact that elevated pressure air separation units are lower in capital than low pressure air separation units, especially in large sizes, and that no recovery of energy from the high pressure nitrogen in an elevated pressure air separation unit is justifiable when recovering that energy cannot, in turn, be used to further reduce equipment costs.